ORDER vii UNGULATA 117 



ground and almost perpendicular, so that the end-phalanges bear the weight 

 of the body. The digitigrade foot, a form quite rare among the ungiilates 

 (Camelidae), has the metapodials elevated from the ground, but all of the 

 phalanges, protected by an elastic pad, rest on the ground. 



2. The metapodials became very much elongated. 



3. Certain metapodials and toes became very M^ell developed and elongated, 

 at the expense of the reduction or disappearance of the lateral metapodials, 

 and frequently also of the lateral earpals and tarsals. 



4. The earpals, tarsals and metapodials, which originally were arranged in 

 part serially in parallel rows, became laterally displaced and wedged into 

 a more or less alternating arrangement. 



5. Certain earpals, tarsals and metapodials, separate in the primitive 

 forms, became fused. 



The dentition of the ungulates is of service for the classification of genera 

 in equally important degree as the structure of their extremities. The 

 primitive Ungulata had a complete set of teeth, all of which, with possibly 

 the exception of the front P, were very closely set together. Owing to the 

 elongation of the jaws, large diastemas were formed between the canines, 

 incisors and anterior premolars. The incisors are always single-rooted and 

 originally conical as in the carnivores, but through differentiation become 

 chisel- or spade-like, developing, at times, even into tusks. Reduction in the 

 incisors occurs, all of these teeth, particularly the upper ones, sometimes 

 disappearing. The upper canines, which originally resembled the canines 

 of carnivores, become either angular tusks or diminish in size, or the upper 

 ones become entirely lost, the lower ones functioning as, and resembling, a 

 fourth incisor (Ruminantia). The upper molars seldom remain tritubercular, 

 and have two secondary cusps (conules) even in this earliest stage. A 

 second large inner cusp (hypocone) is developed from either the metaconule 

 or the cingulum. If the cusps connect so as to form ridges or crescents, they 

 pass from the original condition of bunodont into lophodont or selenodont 

 teeth. Further differentiation occurs through the greater development of the 

 base of the tooth either through the formation of a cingulum, or of enamel 

 folds, or of cement accumulation. As a rule, the lower molars can easily be 

 referred to the primitive tritubercular-sectorial type, with a high tricuspate 

 trigonid and a low, likewise tricuspate talonid. Through loss of the paraconid 

 and the further development, both in height and breadth, of the talonid, 

 quadricuspate teeth result, which may be distinguished from the upper molars 

 principally by reason of being less broad. These, too, may become lophodont 

 or selenodont. The last lower molar has frequently an unpaired large rear 

 cusp or a semicircular ridge. The premolars remain more primitive than the 

 molars in all ungulates. A homoeodont dentition is attained only in very highly 

 specialised forms. The reduction of the cheek teeth begins with the first P, 

 and includes, at the most, but two or three P. 



Originally the cheek teeth of the ungulates were low and brachyodont, 

 the lower ones with two roots and the upper with three. Through the 

 dichotomy of the two roots of the lower molars, and the broad middle root 

 of the upper, teeth with four roots each result. Frequently the crown 

 becomes very high in herbivorous animals, finally converting the teeth into 

 hypsodont or prismatic columnar teeth, which are pushed up in the gums 

 as their surface is worn down through use, and develop roots only later in life. 



